An ink jet recording system is a recording system in which ink liquid droplets are jetted directly on a recording member from very fine nozzles and made to adhere to the recording member to obtain characters and images. This system has not only the advantage that the device to be used has a low noise and good operability but also has the advantage that coloration is easy and common paper can be used as recording members and is therefore widely used. The water resistance and light fastness of printing products are regarded as important and there is an increased demand for pigment type inks as the ink used in ink jet printers these days.
However, it is known to be usually very difficult to disperse pigment type ink at a high density stably in water. In this situation, such technologies have been developed that dispersion stability is accomplished by adding a resin in ink or by allowing a pigment to be included in a resin emulsion. (JP-A 6-306317, JP-A 8-218015, JP-A 8-151544, JP-A8-176488 and JP-A2000-144031). This method, however, has the problem that optical density is decreased because of reduced content of a pigment in ink.
As pigment type inks using carbon black, those using so-called self-dispersible carbon black obtained by forming various hydrophilic functional groups on the surface of raw carbon black to disperse carbon black stably in water have been already known as described in JP-A 10-36727 as the pigment to solve this problem.
Also, techniques using carbon black having a high specific surface area to raise optical density are known as described in WO-A00/63306 and the equivalent JP-A 2002-542368. This discloses a production of self-dispersible ink.
JP-A 10-17808 and the equivalent U.S. Pat. No. 5,919,294 discloses an aqueous ink containing carbon having a specified DBP absorption amount.